Patients with Down's Syndrome often undergo what might be called "accelerated aging" process; they are highly susceptible to infection and certain malignant disease, their immune function undergoes rapid decay in early life, and they often have senile degenerative changes in CNS similar to Alzheimer's Disease. Recently, defective phagocytic function has also been reported in young patients with Down's Syndrome as well as in age "normal" individuals. Age related changes in immune responses have been investigated rather extensively. However, the alterations of phagocytic function associated with aging process have not been fully investigated. Phagocytes (including macrophages) are known to play a major role in host defense against infection and malignant disease. Macrophages are also known to be important cells in the "processing" of antigens and collaborations among the immunocompetent cells. We propose to study in detail age related changes in the immune functions of selected patients with Down's Syndrome. We also seek to obtain evidence for alteration of phagocytic function which may contribute to the impairment of immune response associated with the aging process. We wish to investigate the age related change in cell surface properties, using membrane receptors and electrophoretic mobility of cell as a probe and attempt to correlate these with aging of immune response. Since Down's Syndrome is associated with known chromosal anomaly (Trisomy-21), our studies may provide relevant information as to the mechanism of aging processes at cellular level.